A four-wheel-drive vehicle is equipped with a power transfer unit for transferring power from a front wheel side to a rear wheel side, or from the rear wheel side to the front wheel side. For example, in the case where an engine is mounted on a front portion of a vehicle body, an output torque of an engine is input into a front-wheel differential gear unit (front differential) via a clutch or torque converter, and a transmission. The torque input into the front differential is transferred to right and left front wheels, respectively, through right and left front-wheel axles, and is input into a power transfer unit through a differential casing of the front differential.
The power transfer unit (transfer) comprises a transfer input shaft disposed to extend in a vehicle width (lateral) direction and a transfer output shaft disposed to extend in a vehicle front-rear (longitudinal) direction. A transfer drive gear is provided on an outer periphery of the transfer input shaft, and a transfer driven gear is provided on an outer periphery of the transfer output shaft, wherein the two gears are meshed with each other. The transfer input shaft is a hollow shaft, wherein one of the front-wheel axles is penetrated through the transfer input shaft, and the differential casing is coupled to the transfer input shaft to allow torque from an engine side to be input into the transfer input shaft. The torque input into the transfer input shaft is transferred to the transfer output shaft via the transfer drive gear and the transfer driven gear, and then taken out toward the rear wheel side through a propeller shaft coupled to the transfer output shaft.
Such a power transfer unit involves a problem that, upon a fluctuation in the output torque of the engine, gear rattle occurs between the transfer drive gear and the transfer driven gear, thereby causing noise. With a view to coping with this problem, the following Patent Literature 1 describes interposing a damper mechanism comprising a metal compression coil spring and a rubber member, in a power transfer pathway between a rotary member configured to receive an input of torque from the engine side, and the transfer drive gear. It also describes that a fluctuation in torque from the engine side is absorbed by the coil spring, and a fluctuation in torque from the rear wheel side is absorbed by the rubber member, whereby, in either event, the occurrence of gear rattle between the transfer drive gear and the transfer driven gear can be suppressed.